Finalizing O365/Exchange 2010 migration while keeping an Exchange 2016 on Prem

New users don't need to be created on Exchange. You only need to create the users, then populate them with an email address by adding the address to the proxyAddresses attribute. Once that's done, the user will sync up with O365 (as long as you're using AD Connect), and will get a mailbox added as soon as you add a subscription to the account in O365. This can be done through Exchange 2016 by creating Mail Users, then adding the subscription once the account is synced up.

Regarding the Hybrid config wizard, you can run it on Exchange 2016 to create the send and receive connectors if you wish, but it isn't strictly necessary to do that. Move of the Hybrid settings are organizational and should move forward to Exchange 2016 without many issues, so you can feasibly add Exchange 2016 and decommission 2010.

As an aside, Office 365 has it's own spam/virus filter on mailflow, so it isn't necessary to have an additional solution for that purpose. If you like the features of your spam/virus filter service, you can certainly keep it, but do realize that it's redundant and that you'll end up having to look through two separate Spam filtering systems if things break, so it can cause delays in support down the line.

There are prerequisites you need to complete before you uninstall Exchange 2010

1 Migrate all active onpremise mailbox to cloud (O365)
2 point MX and SPF to O365 and test if mail flow is working as appropriate, ensure no mailboxes except system mailboxes left with onpremise Exchange 2010 servers
3 Install public SSL certificate on Exchange 2016 with all relevant hostnames (autodiscover and mail.domain.com etc)
4 Ensure that your Exchange 2016 is published on internet to allow inbound TCP 25 and 443
5 break the hybrid config which is pointing to Exchange 2010
6 Then Rerun hybrid wizard to map it with Exchange 2016
7 check if your AD connect sync process is working as expected
8 Check if O365 mailbox creation from Exchange 2016 is working as appropriate
9 Once that done, move any remaining Exchange 2010 system mailboxes to Exchange 2016
10 Upgrade all your DLs to Exchange 2016 version
11 Check if any host service  using Exchange 2010 to relay emails, if found replace them with Exchange 2016
12 Delete all mailbox databases from Exchange 2010 and migrate any Global Address List to Exchange 2016
13 shutdown Exchange 2010 server for day or two and ensure that no dependency is left
14 Uninstall Exchange 2010

Thank you for the info. I have everything migrated to O365. all users and calendars are already in exchange online. I have mail flow already going through mimecast and O365 so nothing is going through my on prem servers. all relays are being sent to O365.

I'm still using mimecast as i believe their service is better and we still use them for archiving. I've setup O365 to always allow emails from mimecast and ive locked down the IP's.

I only want to use the 2016 server for management purposes. If this is he case do i need to move system mailboxes and global address lists (i thought )365 was handling that now)? Do i need to be concerned with all the remote mailboxes still in the mail contacts folder?

I dont have public folders and ive already dismounted the mailbox stores on the 2010 server for a few days now.

I did add a new user and populated the email address and O365 did create the user. However, I dont see it on my 2016 server like i do for the users I migrated and I cant add any additonal smtp addresses to it since it says im using dirsync (AD connect). which i why i am keeping a 2016 server on prem.

do i need to follow something like this: REMOVE HYBRID or should i just delete the old mailbox databases and uninstall exchnage on the two servers and then run the hybrid config wizard on my 2016 server so i can see O365?

You want to remove exchange 2010, correct?
Exch 2016 is got installed in co-existence mode with Exch 2010
You don't need onpremise GAL, but your cloud / O365 GAL will rely on onpremise GAL
before uninstalling 2010 server, you need to shift every component to 2016 server like OAB (offline GAL) generation server

I did add a new user and populated the email address and O365 did create the user. However, I dont see it on my 2016 server like i do for the users I migrated and I cant add any additonal smtp addresses to it since it says im using dirsync (AD connect). which i why i am keeping a 2016 server on prem.

How you are creating users with onpremise exchange ?
Are you simply creating AD user and make it mail enable OR are you creating remote mailboxes with onpremise exchange servers?
In any case Exchange 2016 server should be able to locate all onpremise mail users and onpremise mailboxes if any in his console
he cannot locate O365 mailboxes or contacts
U should be able to create remote mailboxes (O365 mailbox in Exchange 2016) from Exchange 2016 onpremise console so that its mail enabled user will remain with onpremise setup and corresponding mailbox will get created in O365
The reason for movement of system mailboxes is to avoid any disruption of exchange 2016 functionality post removal of exchange 2010 as 1st database on exchange 2010 contains system mailboxes and they should not be deleted

Hybrid system is required only if you wanted to move mailboxes back and forth (onpremise to O365 and vice versa) and if you need to exchange free-busy info back and forth else hybrid setup is not required
If you already done all mailbox migration, you can remove hybrid config and also remove exchange 2010 after addressing all dependencies
Once you break hybrid, free-busy sharing is not possible,

Then your Azure AD connect along with Exchange 2016 can be used for user management without hybrid setup

for user i created i just added an email address in the users account using ADUC and gave an E1 license in 365. I can log in to the mailbox but 2016 exchange cant see him like the other users that we migrated from on prem to O365.

So once i move GAL and other items to 2016 and remove the 2010 servers what is the process for making a mailbox for a user? we had some new hires last week where i created the user and added a mailbox to our old 2010 server then used the migration option in O365 to move them over.

What is the process for 2016 exchange? I don't need to share free busy info, nor do i need to move mailboxes from on prem to O365 or vice versa.

EDIT: So I see an option to create an O365 user inside the Exchange 2016 eac. I used that and it created both the AD user and also put the mailbox on O365. If this is the case then I think once i transfer OAB, GAL and virtual directories i can uninstall 2010?

If thats true, then my only other questions is, we use templates to create users since they have the correct DL and security groups. i dont see an option to copy from a template when doing it from the 2016 exchange server.

Unfortunately the vendor ive been using has kind of left me high and dry. I have a call with them on Tuesday but they've failed to call me back a couple times.

I've done most of the migration myself but this last part is confusing. there isn't much info online about it.

so, i did what the first article said and used this: Enable-MailUser -Identity "ksmith" -ExternalEmailAddress ksmith@company.com. It did create the user in O365 but i don't see it on the 2016 server as a mailbox. i see it as a mail contact.

So if i remove all hybrid config, then all new hires will be added as a mail contact and show there and not as a mailbox in 2016? If i keep the hybrid configuration then i would use the second article and keep it as a remote mailbox and then it would show in the 2016 as a mailbox server?

is it possible to remove the current hybrid environment, not create another hybrid for 2016 and still add email users to O365 and have the 2016 server see it so that i can make changes if needed?

if i remove the current hybrid and have to create another for 2016, i notice i need to make the 2016 server a hub transport (or is it edge server in 2016?) for the hybrid config and im trying to avoid that because i dont have a need for any email flow between the 2 sites. only the ability to add and manage email users, DL's and mail contacts.

thank you for the help.

Thank you for the help. I followed this article: https://blogs.technet.microsoft.com/exchange/2012/12/05/decommissioning-your-exchange-2010-servers-in-a-hybrid-deployment/ to remove the trust relationship. I then uninstalled exchange on my mailbox server. I only turned off my 2010 cas/hub just in case but while its off i created a user and then used the command in the first article you mentioned in your last post to enable the email address. I added the O365 license and it created the mailbox. Even though it adds the user on the 2016 server side as a mail contact its a mailbox on O365.I'm able to add the user to DL's and add SMTP addresses.

whew. No where has a good step by step. They tell you it can be done and that it should work without hybrid but no one tells you how to configure it.

EDIT 12/31/17 - If i run this command,: [code] Enable-RemoteMailbox "wjones" -RemoteRoutingAddress "wjones@mydomain.mail.onmicrosoft.com[/code] When AD Connect (DirSync) syncs O365 creates the mailbox and it then shows up as a mailbox in 2016 server

thanks again.